Method and apparatus for providing dispersion compensation

ABSTRACT

A dispersion compensation module (DCM) for compensating dispersion of an optical fiber transmission link is provided. The optical fiber transmission link comprises a transmission fiber and the DCM. The DCM comprises at least first and second dispersion compensating fibers, DCF 1  and DCF 2 , respectively. DCF 1  and DCF 2  each have a dispersion, D 1  and D 2 , respectively, a dispersion slope, S 1  and S 2 , respectively, and a relative dispersion slope, RDS 1  and RDS 2 , respectively. The transmission fiber also has a dispersion, D TransFiber , a dispersion slope, S TransFiber , and a relative dispersion slope, RDS TransFiber . DCF 1  and DCF 2  are selected based on their respective relative dispersion slopes, RDS 1  and RDS 2 , respectively. DCF 1  and DCF 2  have particular, lengths, L 1  and L 2 , respectively. The DCFs are combined with each other and with the transmission fiber and RDS 1  and RDS 2  are such that the combination of the transmission fiber with the combined DCFs results in overall dispersion compensation of the optical fiber transmission link.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to optical fibers and, more particularly,to providing very accurate dispersion compensation over an entire rangeof wavelengths.

BACKGROUND OF THE INVENTION

Dispersion in a glass fiber causes pulse spreading for pulses thatinclude a range of wavelengths, due to the fact that the speed of lightin a glass fiber is a function of the transmission wavelength of thelight. Pulse broadening is a function of the fiber dispersion, the fiberlength and the spectral width of the light source. Dispersion forindividual fibers is generally illustrated using a graph havingdispersion on the vertical axis (in units of picoseconds (ps) pernanometer (nm), or ps/nm) or ps/nm-km (kilometer) and wavelength on thehorizontal axis. There can be both positive and negative dispersion, sothe vertical axis may range from, for example, −250 to +250 ps. Thewavelength on the horizontal axis at which the dispersion equals zerocorresponds to the highest bandwidth for the fiber. However, thiswavelength typically does not coincide with the wavelength at which thefiber transmits light with minimum attenuation.

For example, typical single mode fibers generally transmit best (i.e.,with minimum attenuation) at 1550 nm, whereas dispersion for the samefiber would be approximately zero at 1310 nm. The theoretical minimumloss for glass fiber is approximately 0.16 db/km, and that occurs at thetransmission wavelength of about 1550 nm. Because minimum attenuation isprioritized over zero dispersion, the wavelength normally used totransmit over such fibers is typically 1550 nm. Also, Erbium-dopedamplifiers, which currently are the most commonly used opticalamplifiers for amplifying optical signals carried on a fiber, operate in1530 to 1565 nm range. Because dispersion for such a fiber normally willnot be zero at a transmission wavelength of 1550 nm, attempts areconstantly being made to improve dispersion compensation over thetransmission path in order to provide best overall system performance(i.e., low optical loss and low dispersion).

Many techniques have been used for dispersion compensation, includingthe design and use of dispersion-shifted and dispersion flattenedfibers. Dispersion Compensating Modules (DCMs) have also been used inoptical communications systems for dispersion compensation, especiallyin wavelength division multiplexing (WDM) systems. A number of patentsdescribe various uses of DCMs to compensate dispersion including: U.S.Pat. No. 4,261,639 (Kogelnik et al.); U.S. Pat. No. 4,969,710 (Tick etal.); U.S. Pat. No. 5,191,631 (Rosenberg); and U.S. Pat. No. 5,430,822(Shigematsu et al.). These patents compensate dispersion by insertingDCMs at appropriate intervals along the transmission path. The DCMsusually contain Dispersion Compensating Fiber (DCF) of an appropriatelength to produce dispersion of approximate equal magnitude (butopposite sign) to that of the transmission fiber.

One problem with using the known DCMs to compensate dispersion is thatDCF designs are very sensitive to production tolerances. Therefore, ifthe DCF design is not highly precise, then when the DCF is combined withthe transmission fiber, the resulting transmission link may have toomuch residual dispersion (i.e., dispersion on wavelength channels otherthan the center wavelength channel being compensated). This isespecially true in broadband applications where the transmission ratesmay be, for example, 40 gigabits per second (Gbit/s). Also, once the DCFis produced, only the length of the DCF can be selected to meet thedesired target for dispersion compensation. Moreover, selection of theDCF length (and thus the dispersion of the DCM) should ensure that firstorder and higher order dispersion are compensated.

When compensating for higher order dispersion, it is very important thatthe Relative Dispersion Slope (RDS) of the transmission fiber match theRDS of the DCF (and consequently of the corresponding DCM). For a givenfiber, the RDS is defined as the ratio of the dispersion slope, S, ofthe fiber to the dispersion, D, of the fiber. Thus, the RDS for a givenfiber is equal to S/D for that fiber. For a DCF combined with atransmission fiber, the total dispersion and the total dispersion slopeof the compensated link, D_(LINK) and S_(LINK), respectively, can beexpressed by Equations 1 and 2, respectively, as follows:

D _(Link) =D _(TransmFiber) ×L _(TransmFiber) +D _(DCF) ×L_(DCF)  (Equation 1)

S _(Link) =S _(TransmFiber) ×L _(TransmFiber) +S _(DCF) ×L_(DCF)  (Equation 2)

In Equation 1, D_(TransmFiber) corresponds to the dispersion of thetransmission fiber, L_(DCF) corresponds to the length of the DCF, andD_(DCF) corresponds to the dispersion of the DCF. In Equations 1 and 2,L_(TransmFiber) corresponds to the length of the transmission fiber andL_(DCF) corresponds to the length of the DCF. In Equation 2,S_(TransmFiber) corresponds to the dispersion slope of the transmissionfiber and S_(DCF) corresponds to the dispersion slope of the DCF.

When the dispersion of the system is compensated, i.e., when D_(Link)=0(i.e., when D_(LINK) is set equal to 0 for purposes of calculations),the length of DCF needed to compensate for the dispersion slope and thedispersion of the link can be determined by Equation 3. Because thevalues of the DCF dispersion, the transmission fiber dispersion, and thetransmission fiber link are known, the length of DCF needed is given by:

L _(DCF)=−(D _(TransmFiber) /D _(DCF))×L _(TransmFiber).  (Equation 3)

In order to compensate the link for the dispersion slope, S_(DCF), ofthe DCF itself, the RDS for the DCF and for the transmission fiber mustbe matched such that: $\begin{matrix}{{RDS}_{{Trams}.{Fiber}} = {\frac{S_{{Trams}.{Fiber}}}{D_{{Trams}.{Fiber}}} = {\frac{S_{DCF}}{D_{DCF}} = {RDS}_{DCF}}}} & \left( {{Equation}\quad 4} \right)\end{matrix}$

However, when producing a DCF, the production tolerances inherentlycause variations in the dispersion and in the RDS of the DCF. Dispersionvariations at the center wavelength can be compensated by choosing thecorrect length of DCF, but RDS variations are not compensated.Tolerances on RDS for DCF are typically about ±15%, which can causesignificant residual dispersion at the edges of the transmission band,which is undesirable for the aforementioned reasons.

A technique that uses DCM technology for improving dispersioncompensation over a center wavelength and for reducing residualdispersion on wavelengths at the edges of the transmission band isdisclosed in U.S. Pat. No. 5,781,673 (hereinafter the '673 patent),which is assigned to the assignee of the present invention, and which isincorporated herein by reference in its entirety. This patent disclosesa wavelength division multiplexing (WDM) system in which thetransmission path between a WDM receiver and a WDM transmitter comprisesa transmission fiber of a particular length having a particulardispersion of a particular sign combined with a DCF of a particularlength and having a particular dispersion of opposite sign as that ofthe dispersion of the transmission fiber. This combination ensures thatthe center wavelength of the channel will have a nominally zero overalldispersion.

In order to compensate for the residual dispersion on the otherchannels, the '673 patent discloses adding to the link a dispersionslope compensating fiber (DSCF) of a particular length and having arelatively large negative dispersion slope and a nominally zerodispersion. The dispersion slope of the DSCF is calculated as the sum ofthe residual dispersions on the extreme channels of the transmissionpath divided by the wavelength difference between the extreme channels.

Although the technique disclosed in the '673 patent improves dispersioncompensation in broadband applications, a need to further improvedispersion compensation in various applications, such as broadbandapplications, still exists.

SUMMARY OF THE INVENTION

The present invention provides a dispersion compensation module (DCM)for compensating dispersion of an optical fiber transmission link isprovided. The optical fiber transmission link comprises a transmissionfiber and the DCM. The DCM comprises at least first and seconddispersion compensating fibers, DCF1 and DCF2, respectively. DCF1 andDCF2 each have a dispersion, D1 and D2, respectively, a dispersionslope, S1 and S2, respectively, and a relative dispersion slope, RDS1and RDS2, respectively. The transmission fiber also has a dispersion,D_(TransFiber), a dispersion slope, S_(TransFiber), and a relativedispersion slope, RDS_(TransFiber). DCF1 and DCF2 are selected based ontheir respective relative dispersion slopes, RDS1 and RDS2,respectively. DCF1 and DCF2 have particular lengths, L1 and L2,respectively. The DCFs are combined with each other and with thetransmission fiber and the combination results in overall dispersioncompensation of the optical fiber transmission link.

The present invention also comprises a transmission system comprising atleast first and second dispersion compensation fibers, DCF1 and DCF2,respectively, which are combined in the DCM, and with a transmissionfiber. DCF1 and DCF2 are selected based on their respective relativedispersion slopes, RDS1 and RDS2, respectively, such that when DCF1 andDCF2 are combined, the DCM is provided with an effective relativedispersion slope, RDS_DCM. has an effective RDS that matches themagnitude of the RDS of the transmission fiber, but is of opposite side.When the lengths of each of the DCFs are combined with each other andwith the transmission fiber, RDS1 and RDS2 are such that the combinationof the transmission fiber with the combined DCFs results in overalldispersion compensation of the optical fiber transmission link of thetransmission system.

The present invention also provides a method for performing dispersioncompensation. The method comprises the steps of selecting at least firstand second dispersion compensating fibers, DCF1 and DCF2, respectively,each having a dispersion, D1 and D2, respectively, a dispersion slope,S1 and S2, respectively, and a relative dispersion slope, RDS1 and RDS2,respectively. Once DCF1 and DCF2 and their lengths have been selected,the DCFs are combined with each other and with a transmission fiber. Thetransmission fiber also has a dispersion, D_(TransFiber), a dispersionslope, S_(TransFiber), and a relative dispersion slope,RDS_(TransFiber). DCF1 and DCF2 are selected based on their respectiverelative dispersion slopes, RDS1 and RDS2, such that the combination ofthe transmission fiber with the combined DCFs results in overalldispersion compensation of the optical fiber transmission link.

These and other features and advantages of the present invention willbecome apparent from the following description, drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph illustrating RDS distributions for a first DCF, asecond DCF and the combination of the two versus the percentages of DCMsthat are used to match the RDS of a transmission fiber at 1580nanometers (nm).

FIG. 2 is a graph illustrating the distribution of the RDSs of theresulting combination of the DCFs shown in FIG. 1.

FIG. 3 is a graph illustrating the RDS distribution for a DCF having anegative dispersion slope.

FIG. 4 is a graph 30 illustrating the RDS distribution for the DCF ofFIG. 3 combined with a fiber having a positive dispersion.

FIG. 5 is a graph illustrating the dispersion for DCMs that utilize thesingle fiber solution of FIG. 3.

FIG. 6 is a graph that shows the dispersion of the correspondingcombination DCM of FIG. 4.

FIG. 7 is a flow chart illustrating the method of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with the present invention, it has been determined thatoverall dispersion compensation over a transmission band can be furtherimproved by simultaneously matching the RDSs of the DCM and transmissionfiber and compensating the dispersion slope and dispersion of thetransmission link. The present invention not only improves overdispersion compensation of the transmission link by ensuringsimultaneous compensation of the center wavelength of a broadtransmission band and of all of the other wavelengths of transmissionband.

In accordance with the preferred embodiment of the present invention,multiple DCFs having particular RDSs are combined in a single DCM toobtain an effective RDS for the DCM that matches the RDS of thetransmission fiber. In general, this is accomplished by matching theeffective RDS of the DCM with the RDS of the transmission fiber. Theeffective RDS of the DCM is the RDS produced by combining multiple DCFshaving particular RDSs. Selecting the lengths of the DCFs enables thedispersion slope and the dispersion of the transmission link to becompensated. The manner in which this can be accomplished will first bemathematically described and then graphically demonstrated.

The dispersion and the dispersion slope of a DCM can be obtained fromEquations 5 and 6 below, respectively, as follows:

D _(—) DCM=D1×L 1 +D 2×L 2  (Equation 5)

S _(—) DCM=S 1×L 1 +S 2×L 2  (Equation 6)

In Equations 5 and 6, D_DCM corresponds to the dispersion of the DCM, D1corresponds to the dispersion of a first DCF, L1 corresponds to thelength of the first DCF, D2 corresponds to the dispersion of the secondDCF, L2 corresponds to the length of the second DCF, S_DCM correspondsto the dispersion slope of the DCM, S1 corresponds to the dispersionslope of the first DCF and S2 corresponds to the dispersion slope of thesecond DCF. The total dispersion of the DCM is a sum of the dispersionand dispersion slope of the first DCF (hereinafter “DCF1”) and thedispersion and dispersion slope of the second DCF (hereinafter “DCF2”).The dispersion is in units of dispersion per unit length multiplied bythe DCF length. The dispersion slope of the DCM is the sum of thedispersion slope of DCF1 multiplied by its length and the dispersionslope of the DCF2 multiplied by its length.

Equations 5 and 6 can be re-written as Equations 7 and 8, respectively,as follows:

L 1 =D _(—) DCM/D 1×(RDS 2−RDS _(—) DCM)/(RDS 2−RDS 1)  (Equation 7)

L 2=(D _(—) DCM−L 1×D 1 )/D 2  (Equation 8)

where RDS1 corresponds to the RDS for DCF1, which is=S1/D1 (both knownvalues), RDS2 corresponds to the RDS for DCF2 which is=S2/D2 (both knownvalues), D_DCM=the dispersion of the DCM and RDS_DCM is=S of the DCMdivided by D of the DCM.

The target value D_DCM is known because the DCM dispersion for thedesired system should be equal in magnitude, or almost equal, (but ofopposite sign) to the magnitude of the dispersion of the transmissionfiber, which is known. The other target value for the DCM, RDS_DCM, isalso known because the DCM RDS value should be equal in magnitude, oralmost equal, to the magnitude of the RDS of the transmission fiber. TheRDS of the transmission fiber is also known.

By selecting the RDS1 to be lower than the target value RDS_DCM, and byselecting the RDS2 to be higher than the target value RDS_DCM (orvice-versa), the lengths L1 and L2 of DCF1 and DCF2 that need to becombined to meet the RDS_DCM and D_DCM target values can be determinedfrom the above-stated Equations 7 and 8.

Thus, in accordance with the present invention, the effective target RDSfor the DCM can be obtained such that it matches the RDS of thetransmission fiber by selecting the RDS values of the DCFs that are tobe combined in the DCM in such a way that the target effective RDS forthe DCM is obtained. This ensures that higher order dispersion iscompensated and that no (or very little) dispersion will occur on thecenter wavelength channel or on any of the other wavelength channels ofthe broadband. Of course, the present invention is equally applicable tonon-broadband applications. The determination and selection of thelengths of DCF1 and DCF2 ensure that the dispersion and dispersion slopeof the transmission link will be will be compensated, which takes careof all lower order dispersion.

It is typical for system designers to desire that the residualdispersion be close to, but not precisely, zero. Hence, the termD_(OFFSET) may be introduced to offset the target value of D_DCM asfollows:

D _(—) DCM=−D _(LINK) +D _(OFFSET)  (Equation 9)

Equations 5-8 would be equally applicable in the case where thedispersion of the DCM is to have a preselected offset.

In another embodiment of the present invention, the dispersion of theDCM and of the transmission fiber are not matched by presuming that theDCM RDS matches the RDS of the transmission fiber. Rather, a term(D_(Target)−D_DCM) is determined at, for example, the minimumwavelength, the center wavelength and the maximum wavelength to obtain avalue for the term (D_(Target)−D_DCM) at all three wavelengths. Usingthese values, the values of L1 and L2 are calculated that minimize theterm (D_(Target)−D_DCM) at all three wavelengths. In essence, thiscorresponds to determining the DCF lengths L1 and L2 such that the bestmatch of the DCM dispersion to the target dispersion at the threewavelengths is obtained, which, consequently, corresponds to the bestfit for the entire transmission band. As in the first embodiment, inaccordance with this embodiment, two (or more) DCFs (or more) are beingused to obtain a more accurate match of the DCM dispersion to thetransmission link dispersion. The only difference between theseembodiments is the manner in which the lengths L1 of DCF1 and L2 of DCF2are determined.

FIGS. 1-6 are various graphs that demonstrate the manner in which thepresent invention provides very precise compensation. FIG. 1 is a graph1 illustrating RDS distributions versus the percentages of DCMs that areused to match the RDS of a TW-RS transmission fiber at 1580 nanometers(nm). The graph 1 shows the RDS distributions for DCF1, labeled with thenumeral 2, for DCF2, labeled with the numeral 3, and for the combination4 of DCF1 and DCF2. The average RDS of DCF1 was 0.00965 nm⁻¹. Theaverage RDS of DCF2 was 0.00666 nm⁻¹. The average RDS for thecombination 4 of DCF1 and DCF2 was 0.00795 nm⁻¹, which matches thetarget RDS of the TW-RS transmission fiber at 1580 nm. For thisexperiment, DCF1 had a RDS higher than the target RDS value and DCF2 hada RDS lower than the target value. Thus, it can be seen from the graph 1that by combining the two DCFs into one DCM, the target RDS was veryaccurately obtained by a large percentage of the combination DCMs.

FIG. 2 is a graph 10 illustrating the distribution 11 of the RDSs of theresulting combination DCM (i.e., comprising DCF1 and DCF2) versus thepercentage of DCMs in which the combination of DCF1 and DCF2 was used.By looking at FIGS. 1 and 2, it can be seen that a smaller percentage ofmodules are shown on the vertical axis in FIG. 2 and that the RDS scaleshown on the horizontal axis of FIG. 2 is of much higher resolution thanthe RDS scale of FIG. 1. A comparison of the individual distributionsfor DCF1 and DCF2 shown in FIG. 1 with the combination distribution ofDCF1 and DCF2 shown in FIG. 2 shows that the accuracy of the RDS for thecombination is improved roughly by a factor of 10 when comparedindividually to either the RDS of DCF1 or to the RDS of DCF2.

Also, if a DCF with a negative dispersion slope is combined in the DCMwith a fiber that has a positive dispersion, then it is possible for theRDS of the DCM to be increased above the RDS range of the DCF itself.This can happen if 1) the second fiber has a negative dispersion slope,or 2) a positive dispersion slope, but a lower RDS than that of the DCF.In both cases, the higher RDS for the DCM is obtained by 1) using agreater length of DCF than otherwise would be required, therebyincreasing the absolute dispersion slope of the DCM and 2) by loweringthe dispersion of the DCM to the desired value by using the fiber withthe positive dispersion. The correct lengths for both the DCF and thepositive dispersion fiber can be calculated from the dispersion and RDSsof the two fibers of the DCM. For example, if the DCF has a RDS of 0.01nm⁻¹ at 1550 nm, the effective RDS of the DCM can be increased bycombining the DCF with a standard single mode fiber, which typically hasa dispersion of 17 ps/nm/km and a RDS=0.0035 nm⁻¹ (positive dispersionslope) at 1550 nm. This enables compensation of the dispersion slope oftransmission fibers that simply cannot be effectively compensated by aDCM having a single RDS. This is graphically demonstrated by FIGS. 3 and4.

The graph 20 of FIG. 3 illustrates the RDS distribution 21 for a DCFhaving a negative dispersion slope. FIG. 4 is a graph 30 thatillustrates the RDS distribution 31 for the DCF of FIG. 3 combined witha fiber having a positive dispersion. The average RDS of the combinationDCM of FIG. 4 is approximately 20% higher than the average RDS of theDCF of FIG. 3. Also, the width of the combined DCM RDS distribution isonly about ±2.5%, whereas the width of the RDS distribution of the DCFof FIG. 3 is approximately ±15%. Thus, the dispersion match of the DCMto the transmission link is much more accurate for the combination DCM.

FIG. 5 is a graph 40 that illustrates the dispersion for DCMs thatutilize the single fiber solution of FIG. 3. The dispersion variation atthe target wavelength of 1550 nm (target dispersion of −420 p/nm) isindicated by the spreading out of the dispersion curves 41. FIG. 6 is agraph 51 that shows the dispersion of the corresponding combination DCMof FIG. 4, which indicates that the spreading out of the dispersioncurves 51 is much less than the spreading out of the dispersion curves51 shown in FIG. 5. A comparison of the dispersion curves of FIGS. 5 and6 indicates that the dispersion for the combination DCMs (i.e., the DCMswhere at least two fibers are combined) are highly accurate over theentire transmission bandwidth.

FIG. 7 is a flow chart illustrating the method of the present invention,which applies to both of the above-described embodiments. In general,the method 60 comprises the step of selecting a first fiber and a secondfiber to be combined with each other to provide the DCM with the desiredor necessary compensation properties, as indicated by block 61. Once thefibers that are to be combined have been selected, the lengths of thefirst and second fibers that are to be combined are determined, asindicated by block 63. Then, the lengths of the first and second fibersare combined, as indicated by block 64. The DCM comprising the combinedlengths of the first and second fibers are then combined with thetransmission fiber, as indicated by block 65. The manner in which thefirst and second fibers are selected and the manner in which theirrespective lengths are determined are not limited to any particulartechniques or methodology. The preferred ways for accomplishing thesetasks have been discussed above for purposes of demonstrating theconcepts and principals of the present invention. Those skilled in theart will understand, in view of the discussion provided herein, thatthese determinations can be made in different manners. Also, the mannerin which the combination steps 64 and 65 are, or can be, performed isknown to those skilled in the art. Therefore, a discussion of the mannerin which these tasks are performed will be not be provided herein.

The Polarization Mode Dispersion (PMD) of the DCM is also improved bycombining two or more DCFs, provided the DCFs have equal PMDdistributions. This will inherently introduce an averaging effectbecause it is unlikely that both DCFs will have PMD values from the highend of the PMD distribution scale. Therefore, the PMD of the DCM iseffectively reduced. As will be understood by those skilled in the art,any reduction in PMD is important because the PMD value is an indicatorof the varying time delays for different polarizations of lightpropagating through the fiber. If the PMD is relatively high, the pulsespreading due to these varying delays will be relatively large.Conversely, if the PMD value is relatively low, the pulse spreading dueto these delays will be relatively low. Thus, by improving the PMD,dispersion is further compensated.

As stated above, more than two DCFs can be combined to obtain evenbetter compensation of the higher order dispersion (i.e., compensationof residual dispersion on the wavelengths other than the centerwavelength). For example, two DCFs can be combined to obtain a DCM RDSthat matches the RDS of the transmission fiber, and a third DCF can becombined with the other two DCFs to reduce the curvature of thedispersion curve, which means that the dispersion of the link, if any,is kept constant and thus is predictable.

Another benefit of the present invention is that when two or more DCFshaving particular RDSs are combined to obtain a DCM RDS that matches theRDS of the transmission fiber, the dispersion of the DCM can becontrolled much better to either increase the useable bandwidth for agiven maximum residual dispersion or decrease the residual dispersionfor a fixed bandwidth. Both of these capabilities can greatly improvethe performance of optical communication systems.

In all of the above discussion, it should be understood that shortlengths (under 50 m) of additional types of fibers may be used astransition fibers in the DCM between the DCFs and/or between the DCFsand a transmission fiber to reduce the splice loss of the DCFs to eachother and/or to a transmission fiber. It should be noted that thesefibers are not to be considered as additional DCFs.

In summary, the present invention demonstrates that by combiningmultiple fibers of particular lengths in a single DCM that haveparticular, a combined DCM for the DCM can be obtained that enablestotal and overall compensation to be achieved over the entire bandwidthof the transmission link. By selecting fibers that, when combined,provide an effective RDS value that matches the RDS value of thetransmission fiber, higher order residual dispersion is reduced oreliminated. Once the RDS values for the fibers to be combined are known,the lengths of the fibers of the DCM can be selected such that all lowerorder dispersion is compensated. Thus, the present invention enablessimultaneous compensation of the dispersion and dispersion slope of thetransmission link to be compensated, which is highly desirable,especially in broadband applications where higher order residualdispersion can result in deleterious effects at the edges of thetransmission band. Also, by combining two or more DCFs, the variation inthe dispersion of the combination DCM can be significantly reduced. Inaddition, the present invention lessens the need to make new DCFs tocompensate for a specific type of transmission fiber becausecompensation can be obtained by combining a DCF having a lower RDS witha DCF having a higher RDS. Furthermore, if a DCF having a preselectedRDS is combined with a positive dispersion fiber, the effective RDS ofthe DCM can be increased beyond that of the DCF itself, which allowscompensation of the dispersion slope of transmission fibers thatotherwise would are incapable of being effectively compensated with asingle DCF.

It should be noted that the above-described embodiments of the presentinvention are examples of implementations. Those skilled in the art willunderstand from the disclosure provided herein that many variations andmodifications may be made to the embodiments described without departingfrom the scope of the present invention. All such modifications andvariations are within the scope of the present invention.

What is claimed is:
 1. A dispersion compensation module (DCM) forcompensating dispersion of an optical fiber transmission link, theoptical fiber transmission link comprising a transmission fiber, the DCMcomprising: at least first and second dispersion compensating fibers,DCF1 and DCF2, respectively, DCF1 and DCF2 each having a dispersion, D1and D2, respectively, a dispersion slope, S1 and S2, respectively, and arelative dispersion slope, RDS1 and RDS2, respectively, the transmissionfiber having a dispersion, D_(TransFiber), a dispersion slope,S_(TransFiber), and a relative dispersion slope, RDS_(TransFiber), andwherein DCF1 and DCF2 are selected based on their relative dispersionslopes, RDS1 and RDS2, respectively, DCF1 and DCF2 having particular,lengths, L1 and L2, respectively, the DCFs being combined with eachother and with the transmission fiber, wherein RDS1 and RDS2 are suchthat the combination of the transmission fiber with the combined DCFsresults in overall dispersion compensation of the optical fibertransmission link.
 2. The DCM of claim 1, wherein the RDS1 and RDS2 areselected based on the RDS value of the transmission fiber.
 3. The DCM ofclaim 2, wherein the DCM has a dispersion, D_DCM, a dispersion slope,S_DCM, and a relative dispersion slope, RDS_DCM that are result from thecombined lengths of the DCF1 and DCF2, and wherein the lengths L1 and L2are determined by equations: L 1=D _(—) DCM/D 1×(RDS 2−RDS _(—)DCM)/(RDS 2−RDS 1) L 2=(D _(—) DCM−L 1×D 1)/D 2, wherein D_DCM isapproximately equal in magnitude but opposite in sign to D_(TransFiber)and wherein D_(TransFiber) has a known magnitude and sign, and whereinRDS_DCM is approximately equal in magnitude but opposite in sign toRDS_(TransFiber), and wherein RDS_(TransFiber) has a known magnitude andsign.
 4. The DCM of claim 3, wherein the DCM is implemented in abroadband application in which the combined DCFs and transmission lineare comprised as a transmission link that utilizes a plurality ofwavelengths for transmitting data over the transmission link, andwherein the all of the wavelengths are dispersion compensated byselecting the DCFs such that the RDS_DCM will at least substantiallymatch RDS_(TransFiber) and by selecting proper lengths for L1 and L2. 5.The DCM of claim 4, wherein the DCFs are selected and combined such thatresidual dispersion is almost, but not totally, eliminated over at leastsome of said wavelengths.
 6. The DCM of claim 3, wherein the first andsecond DCFs are selected such that, when combined, RDS_DCM will at leastsubstantially match the RDS_(TransFiber).
 7. The DCM of claim 1, whereinRDS1 is higher than RDS_(TransFiber), and wherein RDS2 is lower thanRDS_(TransFiber).
 8. The DCM of claim 1, wherein the combined DCFs andtransmission line are comprise a transmission link that utilizes aplurality of wavelengths for transmitting data over the transmissionlink and wherein D_DCM is selected so that it does not matchD_(TransFiber) and is calculated for at least two of said plurality ofwavelengths to obtain a D_DCM value for each respective wavelength, thelowest D_DCM value calculated being used to obtain an L1 and an L2 valuefor said at least two of said plurality of wavelengths, the respectiveL1 values for each wavelength being used to obtain a new L1 value, therespective L2 values for each wavelength being used to obtain a new L2value, the new L1 and L2 lengths of said at least first and second DCFs,respectively, being combined with each other and with said transmissionfiber to provide dispersion compensation over said plurality ofwavelengths.
 9. The DCM of claim 1, wherein at least a third DCF, DCF3,is combined with DCF1 and DCF2, respectively, DCF3 having a dispersionD3, a dispersion slope S3, and a relative dispersion slope, RDS3 andwherein DCF1,DCF2 and DCF3 are selected based on their respectiverelative dispersion slopes, RDS1, RDS2, and RDS3, respectively, DCF3having a particular length, L3, the DCFs being combined with each otherand with the transmission fiber, wherein RDS1, RDS2 and RDS3 are suchthat the combination of the transmission fiber with the combined DCFsresults in overall dispersion compensation of the optical fibertransmission link.
 10. A transmission system comprising: at least onedispersion compensation module (DCM); at least first and seconddispersion compensation fibers, DCF1 and DCF2, respectively, DCF1 andDCF2 being combined in the DCM; a transmission fiber, the transmissionfiber having a dispersion, D_(TransFiber), a dispersion slope,S_(TransFiber, and a relative dispersion slope, RDS) _(TransFiber), andwherein DCF1 and DCF2 are selected based on their respective relativedispersion slopes, RDS1 and RDS2, respectively, the DCF1 and DCF2 havingparticular, lengths, L1 and L2, respectively, the DCFs being combinedwith each other and with the transmission fiber, wherein RDS1 and RDS2are such that the combination of the transmission fiber with thecombined DCFs results in overall dispersion compensation of the opticalfiber transmission link.
 11. The transmission system of claim 10,wherein the RDS1 and RDS2 are selected based on the RDS value of thetransmission fiber.
 12. The transmission system of claim 11, wherein theDCM has a dispersion, D_DCM, a dispersion slope, S_DCM, and a relativedispersion slope, RDS_DCM that are result from the combined lengths ofthe DCF1 and DCF2, and wherein the lengths L1 and L2 are determined byequations: L 1 =D _(—) DCM/D 1×(RDS 2−RDS _(—) DCM)/(RDS 2−RDS 1) L 2=(D_(—) DCM−L 1×D 1)/D 2, wherein D_DCM is approximately equal in magnitudebut opposite in sign to D_(TransFiber) and wherein D_(TransFiber) has aknown magnitude and sign, and wherein RDS_DCM is approximately equal inmagnitude but opposite in sign to RDS_(TransFiber), and whereinRDS_(TransFiber), has a known magnitude and sign.
 13. The transmissionsystem of claim 12, wherein the DCM is implemented in a broadbandapplication in which the combined DCFs and transmission line arecomprised as a transmission link that utilizes a plurality ofwavelengths for transmitting data over the transmission link, andwherein the all of the wavelengths are dispersion compensated byselecting the DCFs such that the RDS_DCM will at least substantiallymatch RDS_(TransFiber) and by selecting proper lengths for L1 and L2.14. The transmission system of claim 13, wherein the DCFs are selectedand combined such that residual dispersion is almost, but not totally,eliminated over at least some of said wavelengths.
 15. The transmissionsystem of claim 12, wherein the first and second DCFs are selected suchthat, when combined, RDS_DCM will at least substantially match theRDS_(TransFiber).
 16. The transmission system of claim 10, wherein RDS1is higher than RDS_(TransFiber), and wherein RDS2 is lower thanRDS_(TransFiber).
 17. The transmission system of claim 10, wherein thecombined DCFs and transmission line are comprise a transmission linkthat utilizes a plurality of wavelengths for transmitting data over thetransmission link and wherein DCF1 and DCF2 are selected to ensure thatD_DCM does not match D_(TransFiber) and is calculated for at least twoof said plurality of wavelengths to obtain a D_DCM value for eachrespective wavelength, the lowest D_DCM value calculated being used toobtain an L1 and an L2 value for said at least two of said plurality ofwavelengths, the respective L1 values for each wavelength being used toobtain a new L1 value, the respective L2 values for each wavelengthbeing used to obtain a new L2 value, the new L1 and L2 lengths of saidat least first and second DCFs, respectively, being combined with eachother and with said transmission fiber to provide dispersioncompensation over said plurality of wavelengths.
 18. The transmissionsystem of claim 10, further comprising at least a third DCF, DCF3, iscombined with DCF1 and DCF2, respectively, DCF3 having a dispersion D3,a dispersion slope S3, and a relative dispersion slope, RDS3 and whereinDCF1,DCF2 and DCF3 are selected based on their respective relativedispersion slopes, RDS1, RDS2, and RDS3, respectively, DCF3 having aparticular length, L3, the DCFs being combined with each other and withthe transmission fiber, wherein RDS1, RDS2 and RDS3 are such that thecombination of the transmission fiber with the combined DCFs results inoverall dispersion compensation of the optical fiber transmission link.at least first and second dispersion compensation fibers, DCF1 and DCF2,respectively, DCF1 and DCF2 being combined in the DCM; a transmissionfiber, the transmission fiber having a dispersion, D_(TransFiber), adispersion slope, S_(TransFiber), and a relative dispersion slope,RDS_(TransFiber), and wherein DCF1 and DCF2 are selected based on theirrespective relative dispersion slopes, RDS1 and RDS2, respectively, theDCF1 and DCF2 having particular, lengths, L1 and L2, respectively, theDCFs being combined with each other and with the transmission fiber,wherein RDS1 and RDS2 are such that the combination of the transmissionfiber with the combined DCFs results in overall dispersion compensationof the optical fiber transmission link.
 19. A method for performingdispersion compensation, the method comprising the steps of: selectingat least first and second dispersion compensating fibers, DCF1 and DCF2,respectively, DCF1 and DCF2 each having a dispersion, D1 and D2,respectively, a dispersion slope, S1 and S2, respectively, and arelative dispersion slope, RDS1 and RDS2, respectively; combining alength, L1 of DCF1 with a length L2 of DCF2; and combining the combinedDCFs with a transmission fiber, the transmission fiber having adispersion, D_(TransFiber), a dispersion slope, S_(TransFiber), and arelative dispersion slope, RDS_(TransFiber), and wherein DCF1 and DCF2are selected based on their respective relative dispersion slopes, RDS1and RDS2, wherein RDS1 and RDS2 are such that the combination of thetransmission fiber with the combined DCFs results in overall dispersioncompensation of the optical fiber transmission link.
 20. The method ofclaim 19, wherein RDS1 and RDS2 are selected based on the RDS value ofthe transmission fiber.
 21. The method of claim 20, wherein RDS1 ishigher than RDS_(TransFiber), and wherein RDS2 is lower thanRDS_(TransFiber).
 22. The method of claim 21, wherein the DCM has adispersion, D_DCM, a dispersion slope, S_DCM, and a relative dispersionslope, RDS_DCM, that are result from the combining lengths of the DCF1and DCF2, and wherein the lengths L1 and L2 are determined by equations:L 1=D _(—) DCM/D 1×(RDS 2−RDS _(—) DCM)/(RDS 2−RDS 1) L 2=(D _(—) DCM−L1×D 1)/D 2, wherein D_DCM is approximately equal in magnitude butopposite in sign to D_(TransFiber) and wherein D_(TransFiber) has aknown magnitude and sign, and wherein RDS_DCM is approximately equal inmagnitude but opposite in sign to RDS_(TransFiber), and whereinRDS_(TransFiber) has a known magnitude and sign.
 23. The method of claim22, wherein the first and second DCFs are selected such that, whencombined, RDS_DCM will at least substantially match theRDS_(TransFiber).
 24. The method of claim 23, wherein the DCM isimplemented in a broadband application in which the combined DCFs andtransmission line are comprised as a transmission link that utilizes aplurality of wavelengths for transmitting data over the transmissionlink, and wherein the all of the wavelengths are dispersion compensatedby selecting the DCFs such that the RDS_DCM will at least substantiallymatch RDS_(TransFiber) and by selecting proper lengths for L1 and L2.25. The method of claim 24, wherein the steps of selecting and combiningthe DCFs are such that residual dispersion is almost, but not totally,eliminated over at least some of said wavelengths.
 26. The method ofclaim 19, wherein, during the selection step, the DCF1 and DCF2 areselected to ensure that when DCF1 and DCF2 are combined, D_DCM does notmatch D_(TransFiber), and wherein D_DCM is determined for at least twoof said plurality of wavelengths to obtain a D_DCM value for eachrespective wavelength, the method further comprising: using the lowestD_DCM value to obtain L1 and an L2 lengths for said at least two of saidplurality of wavelengths, the respective L1 lengths for each wavelengthbeing used to obtain a new L1 length, the respective L2 lengths for eachwavelength being used to obtain a new L2 value; and combining the new L1and L2 lengths of DCF1 and DCF2, respectively, with each other and withsaid transmission fiber to provide dispersion compensation over saidplurality of wavelengths.
 27. The method of claim 19, further comprisingthe steps of: selecting at least a third DCF, DCF3, to be combined withDCF1 and DCF2, respectively, DCF3 having a dispersion D3, a dispersionslope S3, and a relative dispersion slope, RDS3; combining DCF3 withDCF1 and DCF2 combined with each other and with the transmission fiber,wherein RDS1, RDS2 and RDS3 are such that the combination of thetransmission fiber with the combined DCFs results in overall dispersioncompensation of the optical fiber transmission link.